Gold deposits of the Bardoc Tectonic Zone: a distinct style of orogenic gold in the Archaean Eastern Goldfields Province,Yilgarn Craton,Western Australia

The Bardoc Tectonic Zone is an ～80 km-long and up to 12 km wide, intensely sheared corridor of Late Archaean supracrustal rocks that is bounded by pre- to syn-tectonic granites in the Eastern Goldfields Province, Yilgarn Craton. This zone has produced over 100 t of gold from a range of deposits, the largest being Paddington (～40 t Au). This shear system is connected along strike to the Boulder – Lefroy Shear Zone, which hosts considerably larger deposits including the giant Golden Mile Camp (>1500 t produced Au). In contrast to the diverse characteristics of gold deposits associated with the Boulder – Lefroy Shear Zone, mineralogical and geochemical data from five representative localities in the Bardoc Tectonic Zone have relatively uniform features. These are: (i) quartz – carbonate veins in competent mafic units with wall-rock alteration characterised by carbonate + quartz + muscovite + chlorite ± biotite + sulf-arsenide + sulfide + oxide + gold assemblages; (ii) arsenopyrite as the dominant sulfur-bearing mineral; (iii) a unique three-stage paragenetic history, commencing with pyrrhotite, and progressing to arsenopyrite and then to pyrite-dominated alteration; (iv) a lack of minerals indicative of oxidising conditions, such as hematite and sulfates; (v) δ³⁴ sulfur compositions of pre- to syn-gold iron sulfides ranging from 1 to 9 ‰; and (vi) a lack of tellurides. These features characterise a coherent group of moderately sized orogenic-gold deposits, and when compared with the larger gold deposits of the Boulder – Lefroy Shear Zone, potentially highlight the petrological and geochemical differences between high-tonnage and smaller deposits in the Eastern Goldfields Province.     

Distribution of orogenic gold deposits in relation to fault zones and gravity gradients: targeting tools applied to the Eastern Goldfields, Yilgarn Craton, Western Australia

A quantitative spatial analysis of mineral deposit distributions in relation to their proximity to a variety of structural elements is used to define parameters that can influence metal endowment, deposit location and the resource potential of a region. Using orogenic gold deposits as an example, geostatistical techniques are applied in a geographic-information-systems-based regional-scale analysis in the high-data-density Yilgarn Craton of Western Australia. Metal endowment (gold production and gold ‘rank’ per square kilometer) is measured in incremental buffer regions created in relation to vector lines, such as faults. The greatest metal tonnages are related to intersections of major faults with regional anticlines and to fault jogs, particularly those of dilatant geometry. Using fault length in parameter search, there is a strong association between crustal-scale shear zones/faults and deposits. Nonetheless, it is the small-scale faults that are marginal or peripheral to the larger-scale features that are more prospective. Gravity gradients (depicted as multiscale edges or gravity ‘worms’) show a clear association to faults that host gold deposits. Long wavelength/long strikelength edges, interpreted as dominantly fault-related, have greater metal endowment and provide a first-order area selection filter for exploration, particularly in areas of poor exposure. Statistical analysis of fault, fold and gravity gradient patterns mainly affirms empirical exploration criteria for orogenic gold deposits, such as associations with crustal-scale faults, anticlinal hinge zones, dilational jogs, elevated fault roughness, strong rheological contrasts and medium metamorphic grade rocks. The presence and concurrence of these parameters determine the metallogenic endowment of a given fault system and segments within the system. By quantifying such parameters, the search area for exploration can be significantly reduced by an order of magnitude, while increasing the chance of discovery.     

Architecture and geodynamic evolution of the St Ives Goldfield,eastern Yilgarn Craton,Western Australia

A new structural evolution consisting of both extensional and contractional events has been defined for the St Ives Goldfield in the south-central Kalgoorlie Terrane of the eastern Yilgarn Craton in Western Australia. These events shaped the development of the fault architecture, which controlled the location of the regional anticlines, the magmatic centres, and the deposition of the Archaean greenstone successions. The fundamental grain of the St Ives Goldfield is north-northwest-trending. This trend is marked by faults which developed during D₁ extension, which was oriented east-northeast–west-southwest. Across these faults we map major stratigraphic changes in the thickness and composition of units, especially of the previously undivided Black Flag Group volcaniclastic rocks. The centre of the St Ives Goldfield is dominated by the Kambalda Anticline. This north-northwest-trending regional fold was likely established early during the D₁ extensional history, and was fully established during subsequent east-northeast-oriented D₂ contraction. The regional anticline is an important architectural element because (1) magmatism and gold mineralising fluids were focussed into this domed region, and (2) deformation was partitioned across the limbs and crest of this structure. The D₃ event involved regional uplift and extension, resulting in the formation of late basins (Merougil Conglomerate locally) and the emplacement of granitoids sourced from a metasomatised mantle wedge (Mafic-type porphyries). The most significant gold event in terms of endowment occurred during D_4b sinistral strike-slip shearing and associated thrusting (e.g., Tramways and Republican thrusts). These thrusts were previously interpreted as the first contractional structures to deform the area (‘D₁’), but are here reinterpreted as relatively late (D_4b). In this D_4b event, the north-northwest-trending faults underwent sinistral strike-slip shearing and were linked across the Kambalda Anticline by accommodation structures represented by generally east- to east-northeast-trending thrusts. Reactivation of D₁ transfer structures may have influenced the location of these later accommodation structures. Late-stage mineralisation during D₅ was the result of dextral strike-slip brittle shearing.     

Scale-integrated architecture of a world-class gold mineral system: The Archaean eastern Yilgarn Craton,Western Australia

World-class mineral systems, such as those found in the Archaean eastern Yilgarn Craton, are the product of enormous energy and mass-flux systems driven by lithospheric-scale processes. These processes can create big footprints or signatures in the lithosphere, which can be observed at a range of scales and via a variety of methods: including geophysics, isotopes, tectonostratigraphy and geochemistry. We use these datasets to describe both the architecture (structure) of world-class gold systems of the Yilgarn Craton and the signatures of their formation. By applying an understanding of the most critical elements of the process, and their signatures, new areas, especially undercover, may be targeted more predictably than before.     

Occurrence of komatiites in the Sandstone greenstone belt,north-central Yilgarn Craton *

Recent geological mapping and exploration drilling has identified widespread but poorly exposed komatiites in the southern part of the Sandstone greenstone belt, which represent the most significant occurrence of komatiites so far recognised in the north-central Yilgarn Craton. Despite serpentinisation and talc – carbonate alteration, relict olivine-cumulate and less common olivine-spinifex textures are preserved. Whole-rock geochemistry indicates the presence of aluminium-depleted and aluminium-undepleted komatiites, both of which are also found in the Forrestania greenstone belt of the south-central Yilgarn Craton.     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造（BIF）有关,可分为2种类型：①深成—表生矿床;②表生—富集矿床,主要分布在尤恩米（Youanmi）地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造(BIF)有关,可分为2种类型:1深成—表生矿床;2表生—富集矿床,主要分布在尤恩米(Youanmi)地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

Numerical modelling of orogenic processes and gold mineralisation in the southeastern part of the Yilgarn Craton,Western Australia

We use numerical modelling codes to simulate aspects of some current hypotheses for the origin of gold deposits and hydrothermal systems in the Yilgarn Craton of Western Australia. In particular, we investigate conceptual models advocating vertically continuous hydrothermal systems as well as those invoking extensive lateral flow and possible links with advection of heat by late orogenic granitic magmatism. Numerical models of part of the Eastern Goldfields Province and Southern Cross Province have been built with FLAC3D, to simulate crustal‐scale coupled interaction between deformation and fluid flow. These illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upwards flow of fluids derived from melting and metamorphism in the deep crust. In some cases, downflow also occurs within the middle crust, at depths where fluid influx might trigger melting if the geothermal gradient were appropriate. The models indicate that tectonic wedging within a layered crust and diverging thrust systems that generate ‘pop‐up’ wedges may be important in facilitating efficient fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. However, the effect of topography appears more important than the precise depth or location of the site of fluid production in the deep crust. The effects of thermal convection and fluid‐fluid interaction have also been numerically modelled for a simplified section across the Kalgoorlie Terrane. Modelling under both hydrostatic and lithostatically overpressured pore‐pressure gradients has effectively delineated domains of convective fluid flow within the middle and upper crust, and has identified two generic sites that are favourable for fluid mixing, notably hangingwall and footwall environments in major shear zones, such as the Bardoc Shear, and in broad antiforms, such as the Goongarrie ‐ Mt Pleasant Antiform. The thermal effect of small plutons embedded in a regional metamorphic regime can cause significant lateral displacement of fluid convection patterns, over distances greater than pluton diameter, as well as more proximal effects on precipitation and dissolution of mineral species. However, these results are highly dependent on the pore‐pressure gradient and the permeability structure of the crust, and require magmatic and metamorphic fluid generation to be precisely timed with respect to deformation, thus reinforcing the dynamic feedback between deformation, magmatism and fluid production and migration.     

4D architecture and tectonic evolution of the Laverton region,eastern Yilgarn Craton,Western Australia

The Laverton region, located in the eastern Yilgarn Craton (EYC) Western Australia, is second only to the Kalgoorlie region for gold endowment. The integration of high-density, potential-field data, regional- and camp-scale seismic reflection data, regional- and mine-scale structural analysis, and geochronologically-constrained stratigraphy, provided new insights into the 4D architecture and tectonic evolution of Laverton region.     

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.     

Predictive mineral discovery in the eastern Yilgarn Craton,Western Australia: An example of district scale targeting of an orogenic gold mineral system

The eastern Yilgarn Craton (EYC) is one of the world's premier gold provinces subject to over a century of mineral exploration. Prolonged interest in the terrane has led to the assembly of multiple world-class data sets suitable for testing district scale targeting methodologies. District scale targeting is concerned with identifying a mineral camp ∼60 km × 60 km in size within a prospective region or province ∼1000 km × 1000 km in size. Exploration at the district scale necessitates the development of predictive exploration models, which can be applied to large regions. Recent advances in the study of the geodynamic evolution and 3D architecture of the EYC, together with an understanding of their interrelationship with the orogenic gold mineral system, has resulted in identification of critical mineralisation processes responsible for the region's rich gold endowment. Here we describe and map these critical processes, using them as a basis for district scale targeting. We relate gold mineralisation to three temporally constrained geodynamic periods, integrated with regional hydrothermal alteration. Unlike many targeting methodologies, this methodology does not incorporate the location of known gold deposits in the analysis, yet it predicted 75% of known gold mineralisation in 5% of the area. The methodology allows critical mineralisation processes to be identified and mapped through time and space. These critical processes are mostly generic and can be applied to other granite–greenstone orogenic gold regions, such as the Abitibi in Canada. An important outcome of this work for the EYC is the identification of a number of new target areas, not known currently for significant gold mineralisation, in what is otherwise thought to represent a mature terrane for gold exploration.     

First record of 1.2 Ga quartz dioritic magmatism in the Archaean Yilgarn Craton,Western Australia,and its significance

Farming of southern bluefin tuna in South Australia currently contributes to more than 30% of the value of the aquaculture production in Australia. This study investigated the natural sedimentary setting of the area designated for this important industry in coastal waters off Port Lincoln, and explored the links between the natural distribution of sediments and potential environmental effects and risks to the industry. Sediments were mostly composed of poorly sorted silts and fine sands, predominantly skeletal remains of carbonate-secreting organisms. The contribution of plankton to the organic matter remaining in the sediments was calculated to be in excess of 80% using concentration-dependent stable-isotope mixing models. An erosional area was identified south of Rabbit Island where sediments contained up to 50% siliciclastic material, grainsize distributions were better sorted and coarser, and organic carbon and total nitrogen contents were very low. In contrast, deeper waters north of Cape Donington were identified as a depocentre for fine sediments, which contained organic matter levels twice those elsewhere in the region despite the extremely high carbonate contents (>80%). The heavier stable isotopic signature of nitrogen suggested that this organic matter comprised a greater fraction of weathered components, probably advected to the area by suspended and bedload transport. This local variability of sediment characteristics in the farming zone suggests that the benthic assimilative capacity of farmed sites will depend on their location. Wastes from pens located south of Rabbit Island in particular are likely to be quickly winnowed out by wave and tidal action. These pens are also less likely to be affected by resuspension of fine sediments that might be associated with unusually severe storms.     

New constraints on the current stress field and seismic velocity structure of the eastern Yilgarn Craton from mechanisms of local earthquakes

The Yilgarn Craton has hosted some of the largest earthquakes within the Australian continent in the last 100 years. Earthquakes have mainly been studied in the western part of the craton, and are thought to result from the reactivation of Precambrian structures in an E–W compressive regional stress field imposed by plate-scale processes. Here we present moment tensor solutions for three recent moderate-sized earthquakes around the town of Kalgoorlie that are inconsistent with E–W compression, but instead suggest E–W extension in the eastern Yilgarn Craton. Waveforms of earthquakes at Boulder (M_W = 4.0, 20 April 2010), Kalgoorlie (M_W = 4.3, 26 February 2014) and Coolgardie (M_W = 3.9, 31 October 2014) were inverted for moment tensors. All three earthquakes were shallow (centroid depth ≤4 km) normal-faulting events that occurred along roughly N–S-striking planes, either with a steep westward or a relatively shallow eastward dip. The robustness of the retrieved mechanisms has been thoroughly tested, employing different earth models, assuming different locations for the earthquakes and using different period bands for the inversion. The fit of synthetic long-period waveforms to the observations was in all cases substantially improved by assuming a two-layered crust with high S wavespeeds (about 3.9–4 km/s) overlying substantially slower material. Since there is independent evidence from active source profiles for a P velocity increase between the upper and lower crust, a large difference in v_p/v_s ratio between upper and lower crust is the only way to explain both lines of evidence. This vertical contrast could represent a dominance of felsic material in the upper crust, and substantially more mafic material in the lower crust. Taken together, our results also appear to imply that the regional stress field is E–W extensive in the Kalgoorlie area, and possibly for the entire Kalgoorlie Terrane. This is contrary to current assumptions from continent-scale stress modelling. That the orientations of rupture planes roughly align with the regional structural grain could indicate that Archean structures are reactivated in response to the current stress field.     

Ages of internal granitoids in the Southern Cross region,Yilgarn Craton,Western Australia,and their crustal evolution and tectonic implications

Southern Cross, where gold deposits are sited in narrow greenstone belts surrounding granitoid domes, was one of the earliest gold mining centres in Western Australia. SHRIMP U–Pb zircon and Pb‐isotope studies of the largest granitoid dome, the Ghooli Dome (80 × 40 km), provide important constraints on the crustal evolution and structural history of the central part of the Archaean Yilgarn Craton, Western Australia, which includes Southern Cross. The north‐northwest‐south‐southeast‐oriented ovoid Ghooli Dome has a broadly concentric foliation that is subhorizontal or gently dipping in its central parts and subvertical along its margins. Foliated granitoids in the dome are dated at ca 2724 ± 5 and 2688 ± 3 Ma using the SHRIMP U–Pb zircon and Pb–Pb isochron methods, respectively. These new data, together with the published SHRIMP U–Pb zircon age of 2691 ± 7 Ma at another locality, 20 km from the centre of the Koolyanobbing Shear Zone, suggest that the Ghooli Dome was emplaced at ca 2.72–2.69 Ga. Because the Ghooli Dome and the other domes, which are enveloped by narrow greenstone belts, are cut by the >650 km‐long and 6–15 km‐wide Koolyanobbing Shear Zone, the ca 2.69 Ga age is interpreted as the maximum age of the last major movement on this structure. The pre‐2.69 Ga history, if any, of the shear zone remains unknown. The shear zone is intruded by an undeformed porphyritic granitoid which has a SHRIMP U–Pb zircon age of 2656 ± 4 Ma. This age is, thus, the minimum age of major movement along this shear zone. Post‐gold mineralisation pegmatitic‐leucogranite from the Nevoria gold mine has a SHRIMP U–Pb zircon age of 2634 ± 4 Ma, with xenocrystic zircon cores of ca 2893 ± 6 Ma, constraining the minimum age of gold mineralisation there to ca 2.63 Ga. The ca 2.72–2.69 Ga granitoids also contain ca 2.98 and 2.78 Ga xenocrystic zircon cores, suggesting an extensive crustal prehistory for their source. Whereas there is a general temporal relationship between the periods of older (ca 3.0 Ga) and younger (ca 2.80 and 2.73 Ga) volcanism and the older (2.98, 2.78 and 2.72–2.69 Ga) granitoid intrusions, there is no known volcanism temporally associated with the 2.65–2.63 Ga granitoid intrusions in the Yilgarn Craton. Other heat sources and/or tectonic processes, required for the generation of these intrusions, are interpreted to be related to a lithospheric delamination event related to continental collision.     

Influence of structural setting on sulphur isotopes in Archean orogenic gold deposits,Eastern Goldfields Province,Yilgarn, Western Australia

The published mean δ³⁴S values of ore-related pyrites from orogenic gold deposits of the Eastern Goldfields Province, Yilgarn Craton lie between −4‰ and +4‰. As for orogenic gold deposits worldwide, most deposits have positive means and a restricted range of δ³⁴S values, but some have negative means and wider ranges of δ³⁴S values. Wall-rock carbonation and back-mixing of similar-source fluids with different fluid pathways can explain some of the more negative δ³⁴S signatures. However, structural setting appears to be the most important factor controlling ore-fluid oxidation state and hence the distribution of δ³⁴S values in gold-related pyrites. Shear-hosted deposits appear to have experienced fluid-dominated processes such as phase separation, whereas stockwork, vein-hosted or disseminated deposits formed under conditions of greater rock buffering. At Victory-Defiance, in particular, negative δ³⁴S values are more common in gently dipping dilational structures, compared to more compressional steeply dipping structures. It appears most likely that fluid-pressure fluctuations during fault-valve cycles establish different fluid-flow regimes in structures with different orientations. Rapid fluid-pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore-fluid oxidation, leading to large, local fluctuations in oxidation state. It is thus not necessary to invoke mixing with oxidised magmatic fluids to explain δ³⁴S signatures indicative of oxidation. In any case, available, robust geochronology in the Eastern Goldfields Province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis. Thus, negative mean δ³⁴S values and large variations in δ³⁴S values of ore-related pyrites in world-class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single, ubiquitous ore fluid in varying structural settings, rather than from the involvement of oxidised ore fluids from a different source. Such signatures are indicative, but not diagnostic, of anomalously large orogenic gold systems. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Characteristics of orogenic-gold deposits in the Northcote district,Hodgkinson Province,north Queensland: implications for tectonic evolution

We report results of metallogenic, structural, petrological and fluid-inclusion studies that characterise the nature of gold mineralisation in the Northcote district of the Hodgkinson Goldfield, one of the major goldfields in the Palaeozoic Hodgkinson Province of northeastern Queensland, Australia. Gold – arsenic and gold – antimony – arsenic deposits in the Northcote district are spatially associated with distinctive northwest-trending faults and shear zones suggesting a strong structural control on their development during several phases of deformation in the Late Devonian to Carboniferous. Field evidence, as well as petrographic, scanning electron microscope and fluid-inclusion analysis of mineralised samples, indicates the occurrence of two stages of gold genesis. Gold associated with the earlier of the two mineralising episodes exists mainly in solid solution with acicular arsenopyrite. SEM analysis indicated that minor sphalerite, galena and tetrahedrite formed during this episode of mineralisation. Gold deposited during the second mineralising episode has a strong association with antimony. Fluid-inclusion studies of quartz vein samples record minimum homogenisation temperatures between 108 and 312°C, and salinities of up to 13 wt% NaCl equivalent. Little to no difference in the nature of the fluids is observed between samples related to the two gold-forming stages. The style of gold mineralisation in the Northcote district is compatible with orogenic-gold deposits that form primarily during compressional and transpressional deformation along convergent plate margins.     

Two cycles of voluminous pyroclastic volcanism and sedimentation related to episodic granite emplacement during the late Archean: Eastern Yilgarn Craton,Western Australia

The thick piles of late-Archean volcaniclastic sedimentary successions that overlie the voluminous greenstone units of the eastern Yilgarn Craton, Western Australia, record the important transition from the cessation in mafic-ultramafic volcanism to cratonisation between about 2690 and 2655 Ma. Unfortunately, an inability to clearly subdivide the superficially similar sedimentary successions and correlate them between the various geological terranes and domains of the eastern Yilgarn Craton has led to uncertainty about the timing and nature of the region's palaeogeographic and palaeotectonic evolution. Here, we present the results of some 2025 U–Pb laser-ablation-ICP-MS analyses and 323 Sensitive High-Resolution Ion Microprobe (SHRIMP) analyses of detrital zircons from 14 late-Archean felsic clastic successions of the eastern Yilgarn Craton, which have enabled correlation of clastic successions. The results of our data, together with those compiled from previous studies, show that the post-greenstone sedimentary successions include two major cycles that both commenced with voluminous pyroclastic volcanism and ended with widespread exhumation and erosion associated with granite emplacement. Cycle One commences with an influx of rapidly reworked feldspar-rich pyroclastic debris. These units, here-named the Early Black Flag Group, are dominated by a single population of detrital zircons with an average age of 2690–2680 Ma. Thick (up to 2 km) dolerite bodies, such as the Golden Mile Dolerite, intrude the upper parts of the Early Black Flag Group at about 2680 Ma. Incipient development of large granite domes during Cycle One created extensional basins predominantly near their southeastern and northwestern margins (e.g., St Ives, Wallaby, Kanowna Belle and Agnew), into which the Early Black Flag Group and overlying coarse mafic conglomerate facies of the Late Black Flag Group were deposited. The clast compositions and detrital-zircon ages of the late Black Flag Group detritus match closely the nearby and/or stratigraphically underlying successions, thus suggesting relatively local provenance. Cycle Two involved a similar progression to that observed in Cycle One, but the age and composition of the detritus were notably different. Deposition of rapidly reworked quartz-rich pyroclastic deposits dominated by a single detrital-zircon age population of 2670–2660 Ma heralded the beginning of Cycle Two. These coarse-grained quartz-rich units, are name here the Early Merougil Group. The mean ages of the detrital zircons from the Early Merougil Group match closely the age of the peak in high-Ca (quartz-rich) granite magmatism in the Yilgarn Craton and thus probably represent the surface expression of the same event. Successions of the Late Merougil Group are dominated by coarse felsic conglomerate with abundant volcanic quartz. Although the detrital zircons in these successions have a broad spread of age, the principal sub-populations have ages of about 2665 Ma and thus match closely those of the Early Merougil Group. These successions occur most commonly at the northwestern and southeastern margins of the granite batholiths and thus are interpreted to represent resedimented units dominted by the stratigraphically underlying packages of the Early Merougil Group. The Kurrawang Group is the youngest sedimentary units identified in this study and is dominated by polymictic conglomerate with clasts of banded iron formation (BIF), granite and quartzite near the base and quartz-rich sandstone units containing detrital zircons aged up to 3500 Ma near the top. These units record provenance from deeper and/or more-distal sources. We suggest here that the principal driver for the major episodes of volcanism, sedimentation and deformation associated with basin development was the progressive emplacement of large granite batholiths. This interpretation has important implication for palaeogeographic and palaeotectonic evolution of all late-Archean terranes around the world.     

华北克拉通北缘三叠纪金矿床地质特征、物质来源与控制因素SCIEI北大核心CSCD

三叠纪是华北克拉通重要的成矿期,三叠纪金矿主要分布在华北克拉通北缘,构成一条近东西向延伸、长约1500km的金成矿带,其中包括乌拉山-大青山、张家口、冀东-辽西、青城子及夹皮沟等多个金矿集中区。华北克拉通北缘三叠纪金矿带的赋矿围岩主要为新太古代-中元古代变质岩系和中生代花岗岩类,金矿床以石英脉型矿化为主,伴随蚀变岩型矿化;中、西段金矿集中区矿化组合表现为金或金钼矿化,东段则以金多金属矿化为主。同位素年代学资料表明,该带金矿主要形成于晚三叠世(240~220Ma);流体包裹体特征表明,成矿流体具中温、低盐度、低密度特征,属于H_(2)O-NaCl-CO_(2)±CH_(4)体系,相分离作用可能是石英脉型金矿金沉淀的主要机制;氢-氧同位素组成进一步表明,成矿流体早期为岩浆水或变质水,后期混入了大气降水。矿石碳、硫、铅、氦-氩同位素及金矿集中区内成矿相关岩体的钕、铪同位素组成表明,华北克拉通北缘三叠纪金矿床的成矿流体和成矿物质均与壳-幔混合作用有关,其中,幔源物质及流体的贡献与区域三叠纪岩浆活动有关,壳源组分的贡献主要来自前寒武纪变质围岩。金矿床形成的构造背景整体上与中亚造山带造山后伸展作用有关:晚三叠世,华北北缘受中亚造山带碰撞后伸展作用的影响,发生岩石圈首次减薄事件,同时发育多条深大断裂及碱性花岗岩-碱性岩的侵入活动,为大规模区域金矿的形成提供了优越条件。金矿带的东段可能同时受扬子克拉通与华北板块碰撞作用的影响,是造成其与中、西段金矿集中区成矿特征有一定差异的主因。     

Review and classification of structural controls on fluid flow during regional metamorphism

Mechanisms for kilometre-scale, open-system fluid flow during regional metamorphism remain problematic. Debate also continues over the degree of fluid flow channellization during regional metamorphism, and the mechanisms for pervasive fluid flow at depth. The requirements for pervasive long-distance fluid flow are an interconnected porosity and a large regional gradient in fluid pressure and hydraulic head (thermally or structurally controlled) that dominates over local perturbations in hydraulic head due to deformation. In contrast, dynamic or transient porosity interconnection and fluid flow accompanying deformation of heterogeneous rock suites should result in moderately to strongly channellized flow at a range of scales, of which there are many examples in the literature. Classification of fluid flow types based on scale and degree of equilibration between fluid and rock, wallrock permeability, and mode of fluid transport contributes to an understanding of key factors that control fluid flow. Closed-system fluid behaviour, with restricted fluid flow in microcracks or cracks and limited fluid–rock interaction, occurs over a range of strains and crustal depths, but requires low permeabilities and/or small fluid fluxes. Long-distance, open-system fluid flow in channels is favoured in heterogeneous rocks at high strains, moderate (but variable) permeabilities, and moderate to high fluid fluxes. Long-distance, broad, pervasive fluid flow during regional metamorphism requires that the rocks are not accumulating high strains and have high permeabilities, low permeability contrasts, and high fluid fluxes. The ideal situation for such fluid flow is in situations where the rocks are undergoing stress relaxation immediately after a major deformation phase. In the mid-crust, fairly specific conditions are thus required for pervasive fluid flow. During active orogenesis, structurally controlled fluid flow (with focused open systems surrounding regions of closed-system behaviour) predominates in most, but not all, regional metamorphic situations, at a range of scales.     

Metallogenesis of Precambrian gold deposits in the Wutai greenstone belt:Constrains on the tectonic evolution of the North China Craton

The Wutai greenstone belt in central North China Craton(NCC) hosts a number of Precambrian gold deposits and ore occurrences. Based on the host rock association, these can be divided into Banded Iron Formation(BIF), meta-volcano-sedimentary and meta-conglomerate types. The two former types formed during ~2.5-2.3 Ga and the third one at ~1.85 Ga. The characteristics of these Precambrian gold deposits are broadly similar with those of the orogenic gold deposits. Based on available geochronological data, here we reconstruct the major tectonic events and their relationship with gold mineralization in the Wutai-Hengshan-Fuping region during Neoarchean to Paleoproterozoic as follows.(1)~2.6-2.5 Ga: widespread intrusion of tonalite-trondhjemite-granodiorite(TTG) magmas in the Hengshan terrane and Fuping continental arc, formation of the Wutai volcanic arc in the southern margin of Hengshan terrane with granitoids emplacement, and the Hengshan-Wutai intra-oceanic arc accretion to the Fuping arc at the end of Neoarchean.(2) ~ 2.5-2.3 Ga: the subduction of Hengshan arc from north leading to persistent magmatism and orogenic gold mineralization.(3)~2.2-2.1 Ga:extension leading to the formation of graben structure in the Wutai and Fuping region, deposition of the Hutuo and Wanzi Group sediments, formation of placer gold through erosion of the orogenic gold deposits.(4)~2.2-2.0 Ga: widespread magmatism in the Wutai-Hengshan-Fuping region.(5)~1.95-1.8 Ga: regional metamorphism associated with collision of the Western and Eastern Blocks of the NCC and associated orogenic gold deposits. The multiple subduction-accretion-collision history and subsequent deep erosion has significantly affected most of the Precambrian gold deposits in the Wutai greenstone belt.